This invention relates to a method of producing aluminum can sheet having low earing characteristics.
In the formation of aluminum can sheet into cup-shaped articles, a characteristic known as "earing" occurs as a scalloped appearance around the top edge of the formed cup. The scallops, or ears as they are more commonly known, are formed during the deep drawing step in the fabrication of the cup and represent an undesirable feature of the article. In aluminum can body stock, the cup is subsequently ironed in multiple rings which can accentuate the scalloped ears. High earing, therefore, can create transport problems with the cup as well as insufficient trim after ironing, clipped ears, and trimmer jams, all of which are unacceptable in aluminum can manufacturing. Thus, it is desired to minimize eating in order to avoid these problems and to increase the quality of the cup. See U.S. Pat. No. 3,318,738, the disclosure of which is incorporated herein by reference.
Commercially acceptable low earing can sheet can be produced using a process involving a multiple-stand hot continuous mill facility. In this process, an aluminum alloy ingot is first processed through a hot reversible or breakdown mill and then is subsequently introduced into a series of 2-6 hot rolling stands, collectively referred to as a hot continuous mill. After this, the hot rolled sheet is cold rolled to produce the final thickness sheet.
The hot continuous mill, while effective in producing low earing characteristics in the aluminum can sheet, represents a major capital expenditure. In addition, this process requires extensive coordination of the individual roll stands within the continuous mill for successful fabrication. Thus, it would be desirable to eliminate the hot continuous mill in order to substantially reduce the capital expense for producing aluminum can sheet, but only if aluminum can sheet can be produced which has low eating characteristics.
It is known to produce aluminum can sheet by eliminating the hot continuous mill. The sheet produced, however, has high earing and that impairs commercial usefulness. This is because it is difficult to control temperature variations throughout the sheet length during each hot rolling pass in a single-stand reversible hot mill. The variations in temperature experienced can result in uncontrolled recrystallization during hot rolling on the single-stand reversible hot mill. As a result of the uncontrolled recrystallization, sheet produced in a single-stand reversible hot mill tends to develop a low cube texture upon annealing. The amount of cube texture that develops upon annealing influences the level of earing that will be exhibited by the final sheet. The low levels of cube texture that typically develop in a single-stand reversible hot mill with a standard preheat produce unacceptably high 45.degree. earing at finish gauge.
Thus, there is a need for an improved method of producing aluminum can sheet that has commercially acceptable eating characteristics in a single-stand reversible hot mill.